1. Field of the Invention
The present invention relates generally to a charge coupled device (CCD). More particularly, the invention relates to an input structure of a CCD which enables direct current reproduction while establishing a coincidence of signal phase at its input and output. The invention also relates to an input structure for a CCD capable of bias level in automatic biasing.
2. Description of the Background Art
Automatic biasing strategy for automatically providing an appropriate bias level, is known technology for CCDs. For example, U.S. Pat. No. 4,625,322, issued on Nov. 25, 1986, to Tukazaki et al. discloses one example of an automatic biasing system for a CCD, which employs a pair of auxiliary registers for adjusting an input bias level for a primary or a main register automatically by feeding back the output of the auxiliary registers to an input of the primary register. Similar technology for automatic adjustment of the input bias level has been disclosed in U.S. Pat. No. 4,139,784 issued on Feb. 13, 1979, to Sauer. Also, European Patent First Publication 01 47 697 published on July 10, 1985 discloses a similar structure for a CCD with automatic biasing.
In these prior proposed automatic biasing systems, the output of the auxiliary register is fed back to an input gate of the primary register, to which an information signal to be transferred through the primary register is supplied. In such CCD, it sometimes becomes necessary to add an inverting amplifier between an information signal input terminal and the input gate. In such case, it becomes necessary to provide a capacity for cutting direct current. Namely, when the information signal is supplied to the input gate of the primary register, good linearity can be obtained. On the other hand, the phase of the output signal of the register becomes inverted from that supplied to the input gate. However, coincidence of phases in the input and output signals tends to be required. In such case, the inverting amplifier becomes necessary to be added. The inverting amplifier can be provided either at the input side of the input gate or at the output side of the output circuit. In case that the inverting amplifier is provided at the output side of the output circuit, a wide dynamic range is required since the inverting amplifier has to invert the signal already amplified by the output circuit. This also requires a complicated circuit construction and thus causes difficulty in designing the circuit. These matters will prevent lowering of cost and reduction of chip size. Therefore, it is easier to provide the inverting amplifier at the input side of the input gate. This causes difficulty in reproduction of the direct current level when automatic biasing technology is introduced. Therefore, it becomes necessary to provide a relatively large capacitor on an order of .mu.F's. Such large capacity may make it difficult to form the capacitor on the IC chip of a CCD.
On the other hand, for such CCD, it sometimes becomes necessary to vary the input bias level. For instance, in case of the video signal delay element, the bias levels for the positive phase video signal and negative phase video signal are significantly different from each other. Therefore, it is preferred to switch the bias level in the automatic biasing system. For achieving this, it is one approach to provide a plurality of automatic biasing circuits and selecting one to provide a variable bias level. However, this clearly requires a wide area on the chip and requires complicated wiring.